Silver halide photography involves the exposure of silver halides with radiation to form a latent image that is developed during photographic processing to form a visible image. Silver halides are intrinsically sensitive only to light in the blue and ultraviolet regions of the spectrum. Thus, when silver halides are to be exposed to other wavelengths of radiation, such as green or red light in a multicolor element, a spectral sensitizing dye is required.
A Helium-neon laser is a typical light source emitting in the red portion of the spectrum (632.8 nm). The advent of solid state diodes that emit red radiation, having wavelengths in the range from 650 to 750 nm, has expanded the useful applications of red-sensitive photographic elements. Because of the wide variety of emission wavelengths, it would be desirable for a red-sensitive photographic element to have broad sensitivity in the red portion of the spectrum. This would allow a single element to be used with light sources, such as lasers and diodes, having a variety of emission wavelengths. Such broad sensitivity can generally be provided by using either a single sensitizing dye that provides broad sensitivity or a combination of sensitizing dyes (usually two) that, by themselves, would provide narrower sensitivity. Many dye combinations have disadvantages, such as poor sensitivity, e.g. due to desensitization, or poor keeping stability, e.g. formation of fog during keeping.
U.S. Pat. No. 5,116,722 discloses a method for forming a silver image by exposing a silver halide emulsion spectrally sensitized to light of 600-690 nm with a trinuclear mero-cyanine dye comprising at least 2 water-solubilizing groups, said trinuclear merocyanine having a benzothiazole or benzoselenazole group. CH patent application No. 677,409 describes merocyanine spectral sensitizing dyes containing at least two acid groups for high contrast silver halide emulsions; a benzothiazole nucleus is preferably present in the structure of the spectral sensitizing dyes.
U.S. Pat. No. 4,717,650 describes a silver halide photographic emulsion having a high spectral-sensitivity in a red-color wavelength range, especially within a wavelength range of from 600 to 690 nm, by use of a benzimidazole pentacarbocyanine spectral sensitizing dye.
U.S. Pat. No. 4,336,323 describes a light insensitive imageable layer comprising a synthetic polymeric binder, a dye containing a quinoline group, a nitrate salt and an acid to obtain a decolourizable imaging system.
Many of the dyes known for spectrally sensitizing photographic silver halide emulsions to the red wavelength of 600 to 690 nm have the disadvantage of having low solubility in water so that they usually have to be dispersed in the emulsions or added thereto in the form of an alcoholic solution. Consequently, they are not removed adequately during the processing. As a result, a considerable residual magenta to blue hue is left in the dried element.
It is an object of the present invention to provide silver halide photographic elements with broad sensitivity in the red portion of the spectrum, having good photographic properties in terms of speed and contrast, and presenting minimal residual stain after processing.